(1) Field of the Invention
The present invention relates to a highly efficient microelectronic process for the formation of optical structures used in the fabrication of color filters and microlens arrays for optoelectronic semiconductor imaging devices which optimize light collection and pixel spectral signal contrast.
(2) Description of Prior Art
Conventional solid-state image sensors for color analog or digital video cameras are typically charge-coupled device (CCD) or complementary metaloxide semiconductor (CMOS) photocdiode array structures which comprise a spectrally photosensitive layer below one or more layers patterned in an array of color filters and above which resides a surface-layer array of microlens elements.
The elementary unit of the imager is defined as a pixel. Each pixel is characterized as an addressable area element with intensity and chroma attributes related to the spectral signal contrast derived from the photon collection efficiency of the microlens array, the spectral transmissivity or reflectivity properties of the color filters, lens array and planarization materials, the spectral response, external quantum efficiency, intrinsic electrical noise properties of the photodetectors and electronic signal processing train. Light and color-sensitive integrated circuits require carefully configured color filters to be deposited on the upper layers of a device in order to accurately translate a visual image into its color components. Conventional configurations may generate a color pixel by employing four adjacent pixels on an image sensor. Each of the four pixels is covered by a different color filter selected from the group of red, blue and two green pixels, thereby exposing each monochromatic pixel to only one of the three basic colors. Simple algorithms are subsequently applied to merge the inputs from the three monochromatic pixels to form one full color pixel. The color filter deposition process and its relationship to the microlens array formation process determine the production cycle-time, test-time, yield, and ultimate manufacturing cost. It is an object of the present invention to teach a color-filter process which optimizes these stated factors.
In addition to the dynamic range and noise of the individual photodetectors, the resolution or fidelity of a CCD or CMOS image is influenced by the overall array size, individual pixel size, spacing, and fill factor. Quantitatively, this performance is described by a wavelength-dependent modulation transfer function (MTF) that relates the two-dimensional Fourier transform of the input image to that of the output. In addition to the obvious effects of pixel aperture and shape, the MTF of an array is affected by spatial carrier diffusion, temporal diffusion, and optical diffraction.
While color image formation may be accomplished by recording appropriately filtered images using three separate arrays, such systems tend to be large and costly. Cameras in which a full color image is generated by a single detector array offer significant improvements in size and cost but have inferior spatial resolution. Single-chip color arrays typically use color filters that are aligned with individual columns of photodetector elements to generate a color video signal. In a typical stripe configuration, green filters arc used on e(very other column with the intermediate columns alternatively selected for red or blue recording. To generate a color video signal using an array of this type, intensity information from the green columns is interpolated to produce green data at the red and blue locations. This information is then used to calculate a red-minus-green signal from red-filtered columns and a blue-minus-green signal from the blue ones. Complete red-minus-green and blue-minus-green images are subsequently interpolated from this data yielding three complete images. Commercial camcorders uses a process similar to this to generate a color image but typically utilize more-complicated mosaic-filter designs. The use of alternate columns to yield color information decreases the spatial resolution in the final image.
The ability of a sensor to capture images in low-level irradiance conditions is critical in applications. The primary attributes of the sensor that determine its ability to capture low-level image light are the geometrical optics of the lens arrangement, fill-factors of lenses and photodiodes, and the photoclectron quantum efficiency and spectral response of the semiconductor in which the photodicdes are fabricated. The quantum efficiency is a measure of the photon-to-electron conversion ratio and, for most CCDs these quantum efficiencies are similar. But, the physical size of the photosensitive area or pixel, coupled with the geometry of the lenses for collecting light and imaging this light onto the useful photosensitive area, create superior or inferior solid-state imagers. Larger pixels use more silicon area which drives up the solid-state imager device manufacturing cost. Instead of increasing the active area, imager manufacturers add extra steps to the manufacturing process to apply a microlens and color filter over each pixel. FIG. 1 exhibits the conventional Prior Art vertical semiconductor cross-sectional profile and optical configuration for color image formation. Microlens 1 residing on a planarization layer which serves as a spacer 2 collects a bundle of light rays from the image presented to the video camera and converges the light into focal cone 3 onto photodiode 8 after passing through color filter 4 residing on planarization layer 5, passivation layer 6, and metallization layer 7. The purpose of the microlens' application in CCD and CMOS imaging devices is to increase imaging sensing efficiency. FIG. 2 illustrates the geometrical optics for incident image light 9 converged by microlens element 10, color filter 11, into focal cone 12, to the focal area 13 within a photoactive area 14 surrounded by a dead or non-photosensitive area 15, wherein the sum of the areas of 14 and 15 comprise the region of the pixel.
Sano et al in U.S. Pat. No. 5,796,154 teaches a solid-state imaging device with a vertical dual-lens structure arranged to redirect the oblique, high-angle off-axis light rays of the image from points occluded by patterned photoshields back onto filling the open apertures of the photodiodes. Sano et al cite their preferred embodiment as a configuration in which the color filter layer(s) is positioned between the upper and lower microlens elements comprising the vertical compound lens. FIG. 3 illustrates the Prior Art in which obliquely incident light 16 is imaged by a single convex microlens 17 and is refractively deviated 18 to miss the open aperture of the photodiode element 20, instead impinging on the pholoshield part 19. In FIG. 4 the Prior Art is shown with a first (lower) convex microlens 21 below the second (upper) microlens 16 to compensate for the high incidence angle of the light rays, placing the image back onto the active area 20 of the sensor.
The problem addressed and claimed to be solved by the referenced patent of Sano et al is that of improving light collection by forming a compound vertical lens with an intermediate color filter between the lens-plane layers.
It is clear, therefore, that lens formation on the topmost layer will suffer from topography and conformability problems imposed by the layers deposited earlier in the microelectronic fabrication process.
An alternative Prior Art approach to the solution of improving image light collection onto the active area of a mosaic array CCD or CMOS image device is provided by Baek in U.S. Pat. No. 5,595,930. As shown in FIG. 5, Baek discloses a method of manufacturing a CCD image sensor wherein recesses 22 are formed in the microelectronic fabrication process, filled with a dye layer, and serve as an array of concave microlenses to collimate the bundle of incident light rays 23 passed by an upper microlens 24 onto the active area of the photodiode imaging array 25.
While the fabrication sequence is different from that of Sano et al, Baek's referenced patent shares the similarity of a vertical compound dual-lens arrangement with an intermediate color filter layer comprised of the dye-filled second concave lens.
It is recognized that both the Baek and Sano referenced patents suffer similar problems of fabricating top-layer microlens arrays over underlayer topographies which are becoming more difficult as pixel sizes shrink.